Which scenario is an example of natural selection?

a.) people kill rattlesnakes that rattle when approached by humans, but people do not kill rattlesnakes that remain silent. the quiet snakes survive and breed.
b.) farmers crossbreed two types of cows to produce a breed of cow that has high quality beef and can tolerate hot climate.

c.) giraffes that have long necks to survive a dry season by eating leaves from the tops of trees. those giraffes breed more successfully that year.

d.) dogs that have a desirable coat color are bred with one another to produce puppies the same coat color